EGU Journal Logos (RGB) Open Access Open Access Open Access Advances in Annales Nonlinear Processes Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Discussions Open Access Open Access Atmospheric Atmospheric Chemistry Chemistry and Physics and Physics Discussions Open Access Open Access Atmospheric Atmospheric Measurement Measurement Techniques Techniques Discussions Open Access Biogeosciences, 10, 4653–4671, 2013 Open Access www.biogeosciences.net/10/4653/2013/ Biogeosciences doi:10.5194/bg-10-4653-2013 Biogeosciences Discussions © Author(s) 2013. CC Attribution 3.0 License. Open Access Open Access Climate Climate of the Past of the Past Discussions The Gela Basin pockmark field in the strait of Sicily (Mediterranean Open Access Open Access Sea): chemosymbiotic faunal and carbonate signaturesEarth System of postglacial Earth System Dynamics Dynamics to modern cold seepage Discussions M. Taviani1,2, L. Angeletti1, A. Ceregato1, F. Foglini1, C. Froglia3, and F. Trincardi1 Open Access 1ISMAR-CNR Istituto di Scienze Marine, Via Gobetti 101, 40129, Bologna, Italy Geoscientific Geoscientific Open Access 2Woods Hole Oceanographic Institution, 266 Woods Hole Rd, Woods Hole, Ma. 02543, USAInstrumentation Instrumentation 3 ISMAR-CNR Istituto di Scienze Marine, Largo Fiera della Pesca 2, 60125, Ancona, Italy Methods and Methods and Correspondence to: M. Taviani ([email protected]) Data Systems Data Systems Discussions Open Access Received: 10 December 2012 – Published in Biogeosciences Discuss.: 22 January 2013 Open Access Revised: 24 April 2013 – Accepted: 23 May 2013 – Published: 12 July 2013 Geoscientific Geoscientific Model Development Model Development Abstract. The geo-biological exploration of a pockmark brachia sp.), and 9609.5 ± 153.5 yr cal BP (L. kazani). One Discussions field located at ca. 800 m below sea level in the Gela basin shell of M. amorpha in core MEDCOR81 (pockmark#6,

◦ 0 00 ◦ 0 00 Open Access (Strait of Sicily, Central Mediterranean) provided a relatively Lat 36 45 38.89 N, Long 14 00 07.58 E, 822Open Access m below sea diverse chemosymbiotic community and methane-imprinted level) provided a sub-modernHydrology age of and 484 ± 54 yr cal BP. Hydrology and carbonates. To date, this is the first occurrence of such a type These ages document thatEarth fluid seepageSystem at this pockmark Earth System of specialised deep-water cold-seep communities recorded site has been episodically sustaining thiotrophic macroben- from this key region, before documented in the Mediter- thic communities since the end ofSciences the Younger Dryas stadial Sciences ranean as rather disjunct findings in its eastern and west- up to sub-recent times. Discussions Open Access ernmost basins. The thiotrophic chemosymbiotic organisms Open Access recovered from this area include empty tubes of the vesti- Ocean Science mentiferan Lamellibrachia sp., loose and articulated shells of Ocean Science lucinids (Lucinoma kazani, Myrtea amorpha), vesicomyids 1 Introduction Discussions (Isorropodon perplexum), and gastropods (Taranis moer- chii). A callianassid decapod (Calliax sp.) was consistently Pockmarks are circular to sub-elliptical crater-like depres- Open Access found alive in large numbers in the pockmark mud. Their sions of variable size (few to many hundredsOpen Access of metres in post-mortem calcified parts mixed with molluscs and subor- diameter) and depth (up to few tens m) typically due to de- Solid Earth dinately miliolid foraminifers form a distinct type of skele- gassing of fluids through fine-grainedSolid Earth unconsolidated marine Discussions tal assemblage. Carbonate concretions display δ13C values sediment under a variety of geologic scenarios (King and as low as −40 ‰ PDB suggesting the occurrence of light McLean, 1970; Hovland, 1989, 1992; Scanlon and Knebel, hydrocarbons in the seeping fluids. Since none of the truly 1989; Kelley et al., 1994; Gay et al., 2003; Trincardi et Open Access Open Access chemosymbiotic organisms was found alive, although their al., 2004; Paull et al., 2007). Pockmarks often occur in skeletal parts appear at times very fresh, some specimens clusters and cover wide areas, representing a common and The Cryosphere 14 widespread feature onThe the Cryosphere continental margins worldwide have been AMS- C dated to shed light on the historical evo- Discussions lution of this site. Lamellibrachia and Lucinoma are two of (Hovland and Judd, 1988). They receive much attention the most significant chemosymbiotic taxa reported from var- especially because escaping fluids are often hydrocarbon- ious Mediterranean cold seep sites (Alboran Sea and Eastern enriched venues in the form of slow seepage, vigorous vent- basin). Specimens from station MEDCOR78 (pockmark#1, ing or even eruptions, at times representing a distinct geo- Lat. 36◦46010.1800 N, Long. 14◦01031.5900 E, 815 m below hazard for offshore constructions and navigation (Newton et sea level) provided ages of 11736 ± 636 yr cal BP (Lamelli- al., 1980; Hovland, 1987; Curzi et al., 1988; Curzi, 2012). Such fluid releases are also considered as a potential co-actor

Published by Copernicus Publications on behalf of the European Geosciences Union. 4654 M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily

Fig. 1. Map showing the shaded relief Digital Terrain Model (DTM of 20 m resolution with 10× vertical exaggeration) of the Gela Basin in the Strait of Sicily. The figure illustrates the location of the pockmarks field discussed in this article (red spots represents major pockmarks); inset, the Mediterranean basin with rectangle showing the same area magnified in the figure. in triggering mass-sediment failure along continental mar- formation about resident chemosymbiotic communities, bio- gins, in conjunction with sea level changes or earthquakes sedimentological processes, authigenic carbonates and seep- (Minisini and Trincardi, 2009). Pockmarks are known to host age temporal variability in this sector of the Mediterranean at times peculiar biota in response to nature of escaping flu- Sea connecting its western and eastern basins. ids, rate of emissions, seepage resilience and water depth (Hovland and Thomsen, 1989; Hovland and Judd, 1988; Taviani, 2011, 2013). Gas charged sediments may in fact 2 Material and methods sustain microbiological communities operating on common The study area was surveyed during cruises HERMES05 compounds in fluids, i.e. hydrogen sulphide and hydrocar- (July 2005), MEDCOR and DECORS December 2009 and bons like methane (Cavanaugh et al., 2006; Dando, 2001; August 2011, respectively, of R/V Urania. Swath bathymetry Duperron, 2010). In turn, such communities could be ex- data were acquired using a Kongsberg Simrad EM710 ploited by specialized metazoans through complex symbiotic multibeam echosounder with a nominal sonar frequency of relationships (see MacDonald et al., 1990; Olu-Le Roy et 70–100 kHz an angular coverage sector of 140◦, a beam al., 2007; Taviani, 2001; Duperron, 2010, with references). width of 1◦ × 1◦ and 800 soundings per ping in dual mode. Pockmarks are also sites were methane-related authigenic Chirp-sonar profiles were obtained using a hull-mounted carbonates could be generated (Hovland et al., 1987; Hov- sixteen-transducer source with a sweep modulated 2–7 kHz land and Judd, 1988; Gontharet et al., 2007). outgoing signal equivalent to a 3.5 kHz profile. In the frame of the EU Hermes and Hermione projects, Bottom samplings were performed using a 1.2 ton gravity the Strait of Sicily in the central Mediterranean was targeted corer, large volume grab and epibenthic hauls (Table 1). as a key area to study sediment instability processes that af- Water-column attributes were measured with a Conductiv- fect continental margins. During such exploration a field of ity/Temperature/Depth profiler (CTD) using a Seabird SBE pockmarks was charted and sampled in detail off the south- 11 PLUS employing the SEASAVE V5.33 software. ern margin of Sicily (Fig. 1). Here we present first data on deep sea pockmarks in the Gela basin resulting in novel in-

Biogeosciences, 10, 4653–4671, 2013 www.biogeosciences.net/10/4653/2013/ M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily 4655

Table 1. Sampling station list of Cruises MEDCOR and DECORS in the Gela Basin.

Station Date Start Lat. Start Long. End Lat. End Long. Start End Gear GGMMSS.XX GGMMSS.XX depth depth MEDCOR-68 19 Dec 2009 36◦45038.2400 14◦00009.0200 820.0 CTD MEDCOR-69 19 Dec 2009 36◦45037.8300 14◦00008.2000 824.0 Grab MEDCOR-70 19 Dec 2009 36◦45016.4200 14◦01001.0700 840.8 Gravity Core MEDCOR-71 19 Dec 2009 36◦45016.0700 14◦00058.7900 36◦45035.0000 14◦00010.8200 840.0 820.0 Epibenthic modified haul MEDCOR-72 19 Dec 2009 36◦45015.1100 14◦01001.1800 36◦45015.6900 14◦01000.6100 842.0 840.6 CTD (POM) MEDCOR-74 19 Dec 2009 36◦45038.8200 14◦00010.3500 36◦44030.700 13◦58048.0600 824.0 850.0 Epibenthic modified haul MEDCOR-75 21 Dec 2009 36◦45048.4100 14◦01015.2900 833.0 Grab MEDCOR-76 21 Dec 2009 36◦45034.5600 14◦00024.8500 820.2 Grab MEDCOR-77 21 Dec 2009 36◦45044.5700 13◦58052.7900 831.7 Grab MEDCOR-78 21 Dec 2009 36◦45040.1300 14◦00008.9000 36◦46010.1800 14◦01031.5900 815.0 811.0 Epibenthic modified haul MEDCOR-80 19 Dec 2009 36◦45031.1800 13◦59054.5700 36◦45040.1000 13◦590.49.7900 820.0 815.0 Epibenthic modified haul MEDCOR-81 19 Dec 2009 36◦45037.8400 14◦00008.2100 832.0 Gravity Core MEDCOR-82 19 Dec 2009 36◦45016.2800 14◦01001.0800 840.9 Grab DECORS-48 8 Aug 2011 36◦46016.8600 14◦00006.8000 816.0 CTD DECORS-49 8 Aug 2011 36◦45039.2400 14◦00006.6400 818.0 Grab DECORS-50 9 Aug 2011 36◦46016.8600 13◦55028.4200 827.0 Grab

Stable isotope analyses were performed at the Mass port complex in the area, has affected the northern mar- Spectrometry Laboratory of the CNR-IGG (Institute for gin of the basin mobilising a volume of sediment in the Geosciences and Earth Resources) Pisa, using a Finnigan- order of 1000 km3 in mid-Pleistocene times (Trincardi and MAT252 mass-spectrometer. The carbonate samples were Argnani, 1990). Following this major basin-wide event, sev- cleansed, powdered and then analysed following standard eral smaller-scales mass failure events impacted the area procedures by reacting carbonate aliquots with 100 % or- up to the Holocene (Minisini et al., 2007) and with recur- thophosphoric acid under vacuum. Results are expressed as rence intervals in the order of a few thousands of years be- ‰ relative to VPDB (Vienna-Pee Dee Belemnite) standard tween successive events (Minisini and Trincardi, 2009). The by assigning a value of +1.95 ‰ (13C) and a value of −2.2 ‰ most recent mass-transport events involved, in various mixes, (18O) exactly to NBS 19 calcite; the analytical error is ±0.1δ two basic distinctive kinds of slope sediments: the Pleis- (Table 2). tocene clinoforms that prograde with increasing slope angle 14C-AMS dating was carried out at the Poznan´ Radiomet- and outbuilding the shelf seaward; and upper Pleistocene to ric Laboratory, Poland (Table 3). Holocene muddy contourite deposits characterised by water- saturated and poorly consolidated sediment (Verdicchio and Trincardi, 2008). In all cases where stacked MTD deposits 3 The pockmark field of Gela appear sandwiched within basin-wide draped units, the lat- ter units appear disrupted by vertical features that have been Pockmarks are common features at many continental mar- ascribed to fluid-escape features from MTDs that became gins in the Mediterranean basin (Stefanon, 1981; Stefanon et rapidly buried and perhaps pressurized (Minisini and Trin- al., 1983; Curzi and Veggiani, 1985; Mazzotti et al., 1987; cardi, 2009). Hovland and Curzi, 1989; Trincardi et al., 2004; Geletti et The Gela basin pockmark field (GBPF) is located in the al., 2008; Coleman et al., 2012; Curzi, 2012; Somoza et al., Strait of Sicily, ca. 20 nm offshore the southern coast of 2012). They have been recently documented also in vari- Sicily in a depth range of 800–900 m (Fig. 1). Firstly detected ous locations of the Strait of Sicily, although most published upon TOBI side scan sonar in 2003 and by low-resolution studies regard relatively shallow-water (ca. 70–200 m) occur- multibeam bathymetric data in 2005, the GBPF was then sur- rences (Savini et al., 2009; Cangemi et al., 2010; Micallef et veyed more in detail in winter 2009 and again in summer al., 2011). Pockmarks do occur in the Gela basin at bathyal 2011 during Hermione cruise DECORS when high resolu- depths (Minisini and Trincardi, 2009, this paper). tion multibeam data were acquired, and additional pockmark Gela basin is the Plio-Quaternary foredeep of the Maghre- fields further west, identified. bian fold-and-thrust belt and is a site of widespread and re- peated mass transport. Gela Slide, the largest mass trans- www.biogeosciences.net/10/4653/2013/ Biogeosciences, 10, 4653–4671, 2013 4656 M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily

Table 2. Stable Isotopes Analysis expressed in the conventional δ notation relative to the Vienna-Pee Dee Belemnite (VPDB) reference standard.

Sample Material 13C δ ‰ σ 13C 18O δ ‰ σ 18O vs PDB vs PDB MEDCOR70 Shell Lucinoma kazani 0.82 ±0.1 2.71 ±0.1 MEDCOR78 Shell Isorropodon perplexum 2.66 ±0.1 2.48 ±0.1 MEDCOR81 Shell Myrtea amorpha −1.23 ±0.1 2.33 ±0.1 MEDCOR81 Shell Isorropodon perplexum 0.16 ±0.1 2.21 ±0.1 MEDCOR81 Claw Calliax sp. −1.84 ±0.1 3.23 ±0.1 MEDCOR70 Mudstone −40.77 ±0.1 4.98 ±0.1 MEDCOR78 Lithified burrow −4.05 ±0.1 3.18 ±0.1 MEDCOR78 Carbonate concretion on shell Lucinoma kazani −37.57 ±0.1 2.86 ±0.1

Table 3. Radiocarbon data from the Poznan´ Radiocarbon Laboratory in Poznan,´ Poland. Radiocarbon years (14C-yr) correspond to the true half-life. Calibrated age ranges reflect the 95.4 % probability window (2σ -error) and were calculated with Calib6.0.1, using the surface marine calibration curve MARINE09 by Reimer et al. (2009).

Code # Sample Species Radiocarbon Age Delta R Two-σ ranges Age cal BP Calibration (14C-yr BP) (yr) (yr cal BP) (yr cal BP) data Poz-38025 MEDCOR78 Lamellibrachia sp. 10 500 ± 200 41 ± 7 11 121–12 343 11 732 ± 611 Marine09 Poz-37995 MEDCOR78 Lucinoma kazani 8960 ± 60 41 ± 7 9456–9763 9610 ± 154 Marine09 Poz-55055 MEDCOR78 Isorropodon perplexum 7920 ± 50 41 ± 7 8082–8583 8333 ± 250 Marine09 Poz-55053 MEDCOR69 Taranis moerchii 7820 ± 130 41 ± 7 7901–8589 8245 ± 344 Marine09 Poz-37994 MEDCOR81 Myrtea amorpha 1090 ± 30 41 ± 7 547–669 486 ± 54 Marine09

The resulting detailed morpho-bathymetric map docu- The largest depression with an internal bulge, resulting ments the existence of discrete clusters of pockmarks in a re- from the coalescence of two pockmarks, was selected as the stricted sector of the Gela basin (Figs. 1–4). Our data do not prime target for bottom sampling. The entire area is a fish- show any clear preferential orientation of these features. All ing ground subjected for many years to a peculiar local fish- pockmarks in the studied field are clustered in seven seafloor ing (mal)practice known as cannizzi that litters the bottom patches for a total area of about 18 km2. The pockmarks by leaving behind scores of dead weights with long plas- presents generally a sub-circular shape, with the exception of tic threads attached (Taviani, 2010). Before proceeding with the pockmark #1 (Fig. 2) which is elliptical shaped, and range grab and core sampling, and CTD casting, it was therefore in diameter from 40 to 310 m, although most of them are be- necessary to clear first the bottom by removing this litter. tween 40 and 100 m and only 11 (Fig. 2) are between 200 m This goal was achieved by using a small modified haul that and 310 m. The bottom of some of these is as much as 20 m in fact successfully entangled some such cannizzi remains below the surrounding seafloor; most however are less then (threads and some weights), the former intensely over grown 6 m deep. In a cross section, extracted from the bathymetric by live cold water corals. A second trawl on the same tra- profile, the pockmarks in most cases appear to be U-shaped; jectory intercepted the bottom collecting many dead shells but four of them are V-shaped and (Fig. 2) show a positive of Lucinoma clams and tubeworms, living ghost shrimps and relief ranging from 4 m to 7 m along their rim. The side slope a few carbonate concretions amidst other benthic organisms ranges from 7◦ to 13◦ reaching a maximum of 16◦. The re- and litter (Fig. 7a). Only at this stage, this large pockmark gional surrounding slope is about 2–3◦. Width to depth ratio and some other pockmarks in the area were tested with grab were calculated for 11 pockmarks from bathymetric profiles sampling and coring, integrating the exploration with CTD crossed near the feature centres. The ratio fell between 11 : 1 casts. Additional bottom samples were acquired in 2011 and and 40 : 1 with most near 18 : 1. There is not consistent rela- on this occasion other pockmarks were also sampled. tion between this ratio and relative size of the pockmark. Noticeably, acoustic signals relatable to the presence of gas entrapped in the sediment (such as BSR) or to active 4 Fauna degassing (plumes) were not detected during both surveys (Fig. 5). Furthermore, CTD profiles show no T –S anomalies. The living component in the benthic macro-and megafauna inside the pockmarks at the time of sampling was rather scarce and mostly represented by sparse bivalves

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Fig. 2. Shaded relief DTM of the study area (5 m resolution and 10 × vertical exaggeration) showing the complex, rough topography punctu- ated by many sub-circular depressions interpreted as pockmarks. The 11 major pockmarks, with diameters greater than 200 m, are numbered from maximum to minimum dimension. Their geo-morphometric characteristics are listed in the table; the bathymetric profiles show the cross section of two main pockmarks showing the different internal geometry, respectively U-shaped and V-shaped with internal positive relief. Furthermore, the image reveals that the major pockmark (Number 1) is formed by two partly coalescent depressions.

Table 4. List of major established (chemosymbiotic bivalves, and tubeworm) and putative (gastropods, callianassid) metazoans associated with pockmark cold seepage at the GBPF.

MEDCOR MEDCOR MEDCOR MEDCOR MEDCOR MEDCOR DECORS DECORS 69 71 76 78 80 82 49 50 Lucinoma kazani x x x Salas and Woodside, 2002 Myrtea amorpha x (Sturany, 1896) Isorropodon perplexum x x x x x x x Sturany, 1896 Taranis moerchii x x x x x (Malm, 1861) Lamellibrachia sp. x x x Calliax sp. x x x x x x

(Ennucula corbuloides, Cardiomya costellata, Kelliella mil- claws ranged between 23 and 128. This is likely a still un- iaris), caprellid amphipods and ghost shrimps. Numerous described species morphologically close to an elusive cal- specimens of a callianassid ghost shrimp (Calliax sp.: Fig. 7f, lianassid taxon reported from shallower settings in the West- Table 4) were found in the pockmark area. In the quanti- ern Mediterranean (de Gaillande and Lagardere, 1966). This tative grab samples its density ranged from 1 up to 7 live species is consistently associated with such type of environ- /0.2 square metre whereas number of loose main ments since a few specimens of this same shrimp have been www.biogeosciences.net/10/4653/2013/ Biogeosciences, 10, 4653–4671, 2013 4658 M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily

Fig. 3. Perspective view of the shaded relief DTM (resolution 5 m and 10 × vertical exaggeration) of the study area showing the distribution of the pockmarks and their geometries; the inset shows a magnification of the highest density pockmarks area with more then 40 pockmarks with variable dimensions (8 of the major ones and up to 30 minor features). also found in active pockmarks in the central Adriatic (Ta- (Corselli and Basso, 1996; Cosel and Salas, 2001; Olu-Le viani et al., 2010). Members of the family Callianassidae Roy et al., 2004; Werne et al., 2004; Duperron et al., 2007; are known to inhabit deep sea reducing habitats worldwide Duperron, 2010; Ivanov et al., 2010a, b; Ritt et al., 2010; (summarised by Komai and Fujiwara, 2012). In the eastern Brissac et al., 2011; Shank et al., 2011; Taviani, 2011; Ro- Atlantic sector close to the Mediterranean, a new subfamily drigues et al., 2012). (Vulcanocalliacinae) and a new species (Vulcanocalliax aru- Another remarkable megabenthic organism found in the tyunovi) have been recently described by Dworschak and GBPF is a siboglinid tubeworm of which only empty tubes Cunha (2007) from a mud volcano in the deep Gulf of Cadiz.´ have been collected by us (Fig. 7a). The morphology of the The macrofaunal death assemblages recovered from the tube resembles Lamellibrachia, a recorded from deep GBPF show a predominance of benthic molluscs over water cold seep sites (Olu-Le Roy et al., 2004; Werne et other components, predominantly decapods, pelagic mol- al., 2004; Duperron et al., 2009; Ritt et al., 2010; Hilario´ luscs, echinoids, serpulids and octocorals, Non-seep organ- et al., 2011; Taviani, 2011), sunken wrecks (Hughes and isms are more diverse in terms of number of species (Ap- Crawford, 2006; Gambi et al., 2011), and hydrothermal vents pendix A), but chemosymbiotic bivalves in the families Lu- (Lott and Zimmerman, 2012) in the basin. Tubes might cinidae, and Vesicomyidae are at places noticeably abundant belong to Lamellibrachia anaximandri (Southward, Ander- (Table 4). Lucinids are represented by the large Mediter- sen and Hourdea, 2011), described from the Anaximan- ranean endemic taxon Lucinoma kazani Salas and Wood- der mud volcano in the eastern Mediterranean (Southward side 2002 (Fig. 7c), and subordinately by Myrtea amorpha et al., 2011), and possibly much widespread in the entire (Sturany, 1896) (Fig. 7e). Vesicomyidae are represented by Mediterranean basin (references in Taviani, 2013). In addi- the Mediterranean endemic Isorropodon perplexum Sturany, tion to these well known deep-water chemosymbiotic meta- 1896 (Fig. 7b), the sole species recorded that far from the re- zoans, we can further list a few other taxa preferentially cent Mediterranean basin. Such bivalves have been recorded coping with reducing environments, i.e. thyasirid (Thyasira, from deep sea reducing habitats in the Mediterranean basin Mendicula, Axinulus, Axinus) and lucinid (Myrtea spinifera)

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Fig. 4. Slope map of the study areas, calculated from the DTM, showing clearly the difference between the side slope of the pockmarks reaching the maximum of 16◦ and the regional surrounding slope about 2–3◦.

Fig. 5. (A) Chirp profile showing the subsurface stratigraphy of the pockmark area; (B) 3-D perspective view of the DTM shows the location of the chirp profile and the sample locations (yellow); red numbers are pockmarks.

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Fig. 6. Chirp sonar profiles across the pockmark-hosting morphologic bulge at the base of the slope showing the top of the last basin-wide mass-transport event (Gela slide?) and the thinning of plane-parallel reflectors (basin-plain fine-grained turbidites and hemipelagites) toward the bulge. Where thinner, this unit is likely easier to breach by the ascending fluids than in other areas. bivalves (Dufour, 2005; Loffler¨ et al., 2005; Taylor et al., pockmarks (Appendix A). Furthermore, the more emblem- 2007; Taylor and Glover, 2010; Brissac et al., 2011; Ro- atic cold seep species (e.g. L. kazani, I. perplexum, Lamel- drigues and Duperron, 2011), and the turrid gastropod Tara- librachia sp.) are not co-occurring in all investigated pock- nis moerchii (Fig. 7d) that has been observed to be par- marks in the Gela basin, showing instead a spatial variabil- ticularly common at such seep sites (Olu-Le Roy et al., ity. On the other hand, spatiotemporal variability in analogue 2004; M. Taviani, unpublished data). Only some pockmarks habitats responding to variability through time and space of were found to contain spoils of truly seep communities that fluids’ composition and flow intensity is a well documented are instead totally absent from the sea bottom outside the trait at modern and past seepage sites (Olu et al., 1997;

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Fig. 7. Chemosymbiotic metazoans recovered from the GBPF: (A) haul whole catch of station MEDCOR 78 (pockmark #1) showing chemosymbiotic bivalves, tubeworms, and carbonate concretions mixed with non-seep fauna and anthropogenic litter; (B) Isorropodon per- plexum from st. MEDCOR71, (C) Lucinoma kazani, theratological shell from st. MEDCOR78, (D) Taranis moerchii from st. MEDCOR69, (E) Myrtea amorpha from st. MEDCOR78, (F) Calliax sp. from st. MEDCOR71.

Jenkins et al., 2007; Olu-Le Roy et al., 2007; Lessard-Pilon composition (Table 2, Fig. 12). The δ13C composition et al., 2010). As it is often the case, the sustainability of of chemosymbiotic bivalve shells display isotopic val- such chemosymbiotic bivalves and siboglinids relies upon ues around between −1.23 and +2.66 ‰ PDB reflect- high sulphide concentrations since their metabolic pathway ing the seawater carbonate reservoir as expected (Cob- is thiotrophic. Thus, they cannot be considered as direct abe, 1998; Lietard and Pierre, 2009). The stable carbon fingerprints of hydrocarbon availability in the seeping fluids composition of a poorly cemented layer in core MED- at the GBPF. COR70, provided instead a δ13C value of −40.77 ‰ PDB. A significantly carbon-depleted composition was also measured for concretioned mudstone encrusting 5 Seep carbonates a dead shell of the chemosymbiotic Lucinoma kazani which provided a δ13C value of −37.57 ‰ PDB. Both – Sediments. The pockmark field affects a predominantly very negative carbon signatures document unquestion- silicoclastic-muddy sector of the southern Sicilian mar- ably that at sometime hydrocarbons were present in the gin. Sampling however reveals the occurrence of bio- seeping fluids of this pockmark field in the Gela basin). genic carbonate production in the form of skeletal com- On the other hand, this region is known to be highly pro- ponents shed primarily by calcified benthic organisms ductive for hydrocarbons (Etiope et al., 2002; Holland et and subordinately by pelagics. A distinct type of car- al., 2003). bonate biogenic sediment is produced here through cold seep factories in the form of medium to coarse skele- The stable carbon range is consistent with authigenic car- tal assemblages enriched in shells of chemosymbiotic bonates formed through the incorporation of carbon derived bivalves, mainly vesicomyids (20–40 %), and calcified from anaerobic methane oxidation (Hovland et al., 1987; parts of callianassid decapods (30–50 %) in the sand to Roberts et al., 1987; Kulm and Suess, 1990; Aharon, 1994; hash fraction (Fig. 8). Secondary components in order Kelly et al., 1995; von Rad et al., 1996; Aloisi et al., 2000, of abundance are miliolid foraminifers (5–10 %), fol- 2002a, b; Elvert et al., 2000; Naehr et al., 2000; Greinert et lowed by echinoids, serpulids, ostracods and octocorals al., 2001; Peckmann et al., 2001; Taviani, 2001; Gontharet et and pelagic shells. This remarkable sediment represents al., 2007; Campbell et al., 2008; Cangemi et al., 2010; Feng a novelty since its genesis is in fact related to chemical and Roberts, 2010; Ivanov et al., 2010a, b; Himmler et al., carbonate factories. 2011; Capozzi et al., 2012). The presence of pyrite might represent a legacy of sulphate reduction in the formative en- – Authigenic carbonates. Besides shells of chemosymbi- vironment of such carbonates (e.g. Gontharet et al., 2007). otic bivalves and tubeworms, sampling from the GBPF Interestingly, the GBPF concretions represent a case of in- produced also carbonate concretions, indurated burrows cipient embedding of chemosymbiotic infaunal and epifau- (Figs. 8 and 9), and cemented mudstones (Fig. 10). nal organisms in authigenic carbonates (Lalou et al., 1992), Microcrystalline pyrite druses were noticed on small eventually turning into macrofossiliferous cold seep rocks darker carbonate concretions. Some carbonates were as those diffused in the Neogene Apennine chain (Taviani, tested for the their stable carbon and oxygen isotopic 1994, 2001, 2011). Finally, the stable oxygen composition www.biogeosciences.net/10/4653/2013/ Biogeosciences, 10, 4653–4671, 2013 4662 M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily

Fig. 8. Residual skeletal assemblage after washing; (A) coarse sand fraction; (B) hash fraction, all from st. MEDCOR69. of bivalves (δ18O values comprised between +2.21 and 7 Discussion −2.71 ‰ PDB) are interpretable as indicative of bottom cold temperatures (cf. Sakai et al., 1992), thus supporting the late glacial ages obtained by radiocarbon dating (see below). The pockmark area investigated in this study is located at the base of the northern slope of Gela basin where Chirp-sonar profile detect the top of the last basin-wide MTD at a depth of 6 Age constraints few tens of metres under a succession of basin-floor muddy turbidites. The pockmarks area are not randomly distributed The absence of distinct signals documenting either active but corresponds to a bulge at the base of slope with a posi- degassing or gas charged sediment close to surface and tive relief of few km2 (Figs. 1 and 3) and the overlying suc- the lack of live chemosymbiotic macro-and megafauna was cession, characterised by draped and onlapping plain-parallel taken as an indication that seepage backing the genesis of reflectors (muddy turbidites?) thins toward the slope and are the GBPF is at present either dormant/episodic or ended. In breached by the pockmarks (Fig. 6). The perfect matching order to unravel this fundamental aspect we examined the of the pockmark field with the slide-related base-of-slope core record, and carbon-dated faunas and carbonate concre- bulge is suggestive of a possible causal relationship. In this tions. Superficial bottom samples and two sedimentary cores view, the fluids emanating through the pockmarks would be were taken from pockmark #1 and #6 (respectively MED- fed from the MTD, acoustically-transparent and possibly gas COR81 and MEDCOR70) to reconstruct the recent history charged deposit. However, in the lack of deeper-penetration of fluid seepage at this place (see Fig. 11). Regarding pock- MCS profiles we cannot rule out the possibility that fluids 14 mark #1, a tube of Lamellibrachia produced an AMS- C come, at least in part, from beneath the MTD rather than age of 11732 ± 611 yr cal BP, while a valve of Lucinoma from it. 14 kazani yielded an AMS- C age of 9610 ± 154 yr cal BP. The genesis of the GBPF under study is thus not fully Isorropodon perplexum and Taranis moerchii shells from the understood yet but a potential link between defluidization same pockmark yielded slightly younger but roughly compa- (pockmarks) and a main motif of this part of the continen- rable ages of 8333 ± 250 yr cal BP and 8245 ± 344 yr cal BP tal margin (mass sediment failure) merits it to be consid- respectively. A valve of Myrtea amorpha from sediment ered. In fact, this cluster of pockmarks in the Gela basin 14 core MEDCOR70 (pockmark #6) provided an AMS- C age are just located distally in front of a complex made up by of 484 ± 54 yr cal BP. Although potentially somewhat influ- stacked multiple submarine mass-transport deposits of late enced by carbon sourced from seeping fluids, our radiocar- Pleistocene age (Minisini et al., 2007). These composite units bon ages clearly indicate that seepage activity in the pock- are by large buried and only a few are exposed at present marks is lasting, albeit not necessarily continuatively, since on the seabottom, the best examples of which are the “twin the end of the last glacial epoch (Younger Dryas stadial) up slides” whose multi-phase emplacement occurred since the to sub-recent times. last Post-Glacial (Minisini et al., 2007; Minisini and Trin- cardi, 2009). Pore fluid gradients, resulting in fluid migra- tion and eventual expulsion onto the sea bottom, are sug- gested to be a potential co-driving factor, although not nec- essarily the dominant, in triggering episodic mass-transport

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Fig. 9. Carbonates from the GBPF: (A–D) authigenic concretions (cemented matrix) embedding skeletal biosomes of seep-related metazoans, all from st. MEDCOR78: (A–B) Calliax, (C–D) Isorropodon; (E) indurated callianassid? burrow, armoured with chemosymbiotic-organisms bioclasts; (F) cemented matrix embedding a reworked black-stained shallow-water foraminifer (Elphidium crispum); (G) cemented mudstone layer at 210–230 cm in core MEDCOR70.

Fig. 10. Incipient methane-driven cementation of matrix on valves of chemosymbiotic bivalves from GBPF, precursors of seep-limestones: (A) Lucinoma kazani encrusted by carbon-depleted concretion (δ13C – 37.57 ‰ PDB, see text), (B) Isorropodon perplexum, almost com- pletely embedded in mudstone.

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Fig. 11. Sedimentary cores MEDCOR81 (pockmark #1), and MEDCOR70 (pockmark #6): photographic (left) and (right) lithostratigraphic logs reporting main chronological, petrographic, isotopic and faunal features.

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comyids (Isorropodon), gastropods (Taranis) and callianas- sid decapods (Calliax) that are almost ubiquitous in our sam- ples. Such homogeneity is not at all surprising once we consider the overall reduced variability of cold seepage at the GBPF when compared to larger and heterogenous seep sites as those observable elsewhere in the ocean (Olu et al., 1997; Sibuet and Olu-Le Roy, 2002; Olu-Le Roy et al., 2007; Cordes et al., 2010), or even in the Mediterranean itself (Olu- Le Roy et al., 2004). Such variability often reflects primarily variations in the fluid regimes (Teichert et al., 2003). Nev- ertheless, minor faunal differences are equally observable at the GBPF such as, for example, the comparable rarity of the large chemosymbiotic clam L. kazani or of the large vesti- mentiferan Lamellibrachia which are not recorded thus far from more than two pockmarks. The GBPF is midway between the Eastern Mediterranean and Alboran basins where a variety of cold seep habitats is exploited by macro-and megabenthic organisms as those found in the study area (e.g., Lucinoma, Isorropodon and Lamellibrachia). Thus, it represents a relevant site to be con- Fig. 12. Stable isotope compositions of bivalve shells (core MED- sidered in the crucial and poorly-known issue of connectivity COR 81), callianassid claw and authigenic carbonates all from st. patterns among geographically disjunct but compositionally MEDCOR70 and MEDCOR78 expressed in the conventional δ no- similar deep sea chemosynthetic sites (e.g., Young, 2003; tation relative to the PDB reference standard. Tyler and Young, 2003; Olu-Le Roy et al., 2007; Marcus et al., 2009; Mullineaux et al., 2010). events that shape this region (Minisini et al., 2007; Min- isini and Trincardi, 2009). The GBPF under scrutiny here lies 8 Conclusions ca. 5 nm away to the west of the “twin slides” and does not 1. The Gela basin deep-water pockmark field host- appear to have contiguity with such features, suggesting that ing Mediterranean-emblematic thiotrophic chemosym- no causative link between the two phenomena (twin-slide- biotic macro-and megafauna (Lucinoma, Isorropodon, slumping and pockmark-fluid expulsion) does exist. A series Lamellibrachia) is relevant as it is the first such site in of still-unexplored pockmarks has been, however, identified this key-region of the Mediterranean Sea transitional to at the toe of the “twin slides” (Minisini and Trincardi, 2009). its eastern and western basins; It is predictable that they may in principle contain similar faunas and carbonates as their counterparts. The multiswath 2. the episodic occurrence of hydrocarbons in the seeping topographic map suggests the subsurface existence of now- fluids is demonstrated by the occurrence of carbonate draped lobate bodies northwards of the pockmarks, that in concretions and mudstones with very depleted stable all likeness represent buried mass-transport deposits com- carbon isotopic composition; parable to those described by Minisini et al. (2007) from this general area. The mise-en-place process of the slump- 3. the quite different ages provided by organisms from in- ing units may in theory cause squeezing out of entrapped dividual pockmarks suggest that seepage activity in the fluids or, otherwise, the under pressuring of fluids related area is not a uniformly-distributed and steady event but to sliding off sediment that may be conducive to gas re- differs in space and time; lease pockmarking the bottom. Release of hydrocarbons has 4. dating of chemosymbiotic Lamellibrachia tubeworms been invoked as a likely mechanism to trigger submarine documents that these biota were thriving since the end slumps (Carpenter, 1981 and many others). On-land, similar of the last glacial epoch (Younger Dryas stadial) and up causative scenarios have been suggested to account for lo- to very recent times; calised Miocene chemosymbiotic faunal assemblages and/or authigenic carbonates associated with slumped bodies in the 5. a possible causative link between seepage with related Apennine chain (Berti el al., 1994; Conti and Fontana, 2002; pockmark genesis, and mass-transport processes is ten- Lucente and Taviani, 2005). tatively suggested but still undemonstrated. The GBPF shows a certain degree of homogeneity re- garding the chemosymbiotic assemblages inhabiting indi- vidual pockmarks. This is especially true for small vesi- www.biogeosciences.net/10/4653/2013/ Biogeosciences, 10, 4653–4671, 2013 4666 M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily

Appendix A

Table A1. Benthic and pelagic molluscs recovered from the GBPF during cruises MEDCOR and DECORS: taxa of proven or putative chemosymbiotic type are bold.

MEDCOR MEDCOR MEDCOR MEDCOR MEDCOR MEDCOR DECORS DECORS 69 71 76 78 80 82 49 50 Benthic Mollusc Cardiomya costellata (Deshayes, 1833) x x x x x x x Cuspidaria rostrata (Spengler, 1793) x x x Thracia sp. x Lucinoma kazani Salas and Woodside, 2002 x x x Myrtea sp. x Myrtea amorpha (Sturany, 1896) x Myrtea spinifera (Montagu, 1803) x x x Axinulus croulinensis (Jeffreys, 1847) x x Axinulus eumyarius (M. Sars, 1870) x x Axinus grandis (Verrill and Smith [in Verrill], 1885) x x x x x Mendicula ferruginosa (Forbes, 1844) x x x Thyasira sp. x x x Thyasira succisa (Jeffreys, 1876) x x x Corbula gibba (Olivi, 1792) x x Xylophaga sp. x x x Acanthocardia (Gray, 1851) x Parvicardium sp. x x Kurtiella cf. bidentata (Montagu, 1803) x Kelliella miliaris (Philippi, 1844) x x x x x x Isorropodon perplexum Sturany, 1896 x x x x x x x Waisiuconcha sp. x Tellimya tenella (Loven,´ 1846) x Abra longicallus (Scacchi, 1835) x x x x x x x x Timoclea ovata (Pennant, 1777) x Venus sp. x Bathyspinula excisa (Philippi, 1844) x Katadesmia cuneata (Jeffreys, 1876) x Malletia obtusa (G. O. Sars, 1872) x Saccella commutata (Philippi, 1844) x Microgloma sp. x Yoldiella curta Verrill and Bush, 1898 x Yoldiella lucida (Loven,´ 1846) x x x x x Yoldiella nana (M. Sars, 1865) x x Yoldiella seguenzae Bonfitto and Sabelli, 1995 x x x x x x x Yoldia minima G. Seguenza, 1877 x Ennucula sp. x Ennucula aegeensis (Forbes, 1844) x x x Ennucula corbuloides (Seguenza, 1877) x x x x x x x Nucula sulcata Bronn, 1831 x x Nucula tumidula Malm, 1861 x x x Asperarca nodulosa (O. F. Muller,¨ 1776) x x Bathyarca pectunculoides (Scacchi, 1835) x x x x x x Limatula gwyni (Sykes, 1903) x Limatula subauriculata (Montagu, 1808) x x x x Notolimea crassa (Forbes, 1844) Parvamussium sp. x Pectinidae sp. x Bittium sp. x Turritella communis Risso, 1826 x x x Epitonium sp. x Haliella stenostoma (A. Adams, 1861) x Triphoridae sp. x Aclis attenuans Jeffreys, 1883 x Melanella sp. x sp. x x fusca (Blainville, 1825) x sp.1 x Alvania sp.2 x Alvania cimicoides (Forbes, 1844) x x x x x x x Alvania elegantissima (Monterosato, 1875) x x x x Alvania subsoluta (Aradas, 1847) x x x x x x x Alvania testae (Aradas and Maggiore, 1844) x

Biogeosciences, 10, 4653–4671, 2013 www.biogeosciences.net/10/4653/2013/ M. Taviani et al.: The Gela Basin pockmark field in the strait of Sicily 4667

Table A1. Continued.

MEDCOR MEDCOR MEDCOR MEDCOR MEDCOR MEDCOR DECORS DECORS 69 71 76 78 80 82 49 50 Benthonella tenella (Jeffreys, 1869) x x x x x x x x Aporrhais serresianus (Michaud, 1828) x x Galeodea echinophora (Linnaeus, 1758) x Amphissa acutecostata (Philippi, 1844) x x x Fusinus rostratus (Olivi, 1792) Drilliola emendata (Monterosato, 1872) x (Calcara, 1841) x x Spirotropis modiolus (de Cristofori and Jan, 1832) x Mangelia serga (Dall, 1881) x x Pleurotomella gibbera Bouchet and Waren,´ 1980 x x x x x Taranis moerchii (Malm, 1861) x x x x x Teretia teres (Reeve, 1844) x Granulina gofasi Smriglio & Mariottini, 1996 x x Pagodula echinata (Kiener, 1840) x x x x x x x Trophonopsis barvicensis (Johnston, 1825) x x x Bathysciadium xylophagum Waren´ and Carrozza in Waren,´ 1997 x x Acteon pusillus (Forbes, 1844) x Crenilabium exile (Jeffreys, 1870) x x x x Eulimella sp. x Syrnola sp. x Colpodaspis sp. x Diaphana lactea (Jeffreys, 1877) x x x Philine sp. x Philine scabra (Muller,¨ 1784) x Philine monterosatoi (Sykes, 1905) x x Roxania monterosatoi Dautzenberg and H. Fischer, 1896 x x x x Fissurisepta granulosa Jeffreys, 1883 x Putzeysia wiseri (Calcara, 1842) x x Dikoleps sp. x Polyplacophora Hanleya sp. x Scaphopoda Antalis agilis (M. Sars in G.O. Sars, 1872) x x x x x Dentalium sp. x Entalina tetragona (Brocchi, 1814) x x Pelagic Mollusc Janthina sp. x Carinaria lamarckii Blainville, 1817 x Atlanta brunnea J. E. Gray, 1850 x x x Atlanta peronii Lesueur, 1817 x x x x x x x x Heterobranchia Cavolinia inflexa (Lesueur, 1813) x x x x x Cavolinia tridentata (Forsskal˚ in Niebhur, 1775) x x x x x x Cavolinia uncinata (Rang, 1829) x x x x x Diacria trispinosa (Blainville, 1821) x x x x Clio pyramidata Linnaeus, 1767 x x x x x Creseis clava (Rang, 1828) x x x x x Hyalocylis striata (Rang, 1828) x x x x Styliola subula (Quoy and Gaimard, 1827) x x x x x x x x Heliconoides inflatus (d’Orbigny, 1834) x x x x x Limacina sp. x x Limacina bulimoides (d’Orbigny, 1834) x x Limacina trochiformis (d’Orbigny, 1834) x x x Peracle diversa (Monterosato, 1875) x x x x x Peracle reticulata (d’Orbigny, 1834) x x x

Acknowledgements. We are indebted to officers, crew, and col- and RITMARE (Ricerca italiana per mare) both funded by the leagues for their skilful cooperation during the HERMES05, Italian Ministry of Research and Technology (MIUR). We are MEDCOR and DECORS Cruises supported by the FPVI Integrated grateful to two anonymous referees for their useful comments. This Projects HERMES (GOCE-CT-2005-511234-1) HERMIONE is ISMAR-CNR scientific contribution no. 1778. (grant agreement no: 226354). The article is also a contribution to the FP-VII-7 COCONET (Grant agreement no: 287844) of Edited by: R. Danovaro the European Community, and to the national projects PRIN “Carbonate conduits linked to hydrocarbons enriched seepages”

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